The use of tracers to obtain information about an oil reservoir and/or about what is taking place therein has been practiced for several decades and has been described in numerous documents. Primarily tracers have been used to monitor fluid paths and velocities. More than one tracer substance may be used concurrently. For instance U.S. Pat. No. 5,892,147 discloses a procedure in which a plurality of different tracer substances are placed at respective locations along the length of a well penetrating a reservoir. The tracer substances are placed at these locations during completion of the well before production begins. The tracer at each location is either attached to a section of pipe before it is placed at that location or is delivered into the location whilst perforating casing at that location. When production begins, monitoring the proportions of the individual tracers in the oil or gas produced by the well permits calculation of the proportions of oil or gas being produced from different zones of the reservoir.
Distinctive chemicals which can be detected in high dilution, such as fluorocarbons, dyes or fluorescers have been used as tracers. Genetically coded material has been proposed (and WO2007/132137 gives a method for detection of biological tags). Radio-isotopes have frequently been used as tracers. Society of Petroleum Engineers paper SPE109,969 discloses the use of materials which can be activated to become short lived radio-isotopes.
Hydraulic fracturing is a well established technique for stimulating production from a hydrocarbon reservoir. Typically a thickened, viscous fracturing fluid is pumped into the reservoir formation through a wellbore and fractures the formation. Thickened fluid is then also used to carry a particulate proppant into the fracture. The fracturing fluid is subsequently pumped out and hydrocarbon production is resumed. As the fracturing fluid encounters the porous reservoir formation a filtercake of solids from the fracturing fluid builds up on the surface of the rock constituting the formation. After fracturing has taken place a breaker (which is usually an oxidizing agent, an acid or an enzyme) may be introduced to break down this filter cake and/or to reduce the viscosity of the fluid in the fracture and allow it to be pumped out more effectively.
Tracers have been used in connection with hydraulic fracturing, mainly to provide information on the location and orientation of the fracture, as for instance in SPE 36675 and U.S. Pat. No. 3,987,850. U.S. Pat. No. 3,796,883 describes a further use of radio-active tracers to monitor the functioning of a well gravel pack.
It is known to associate tracers with a carrier material as particles from which the tracer is released after those particles are placed within a subterranean reservoir. For instance U.S. Pat. No. 6,723,683 uses starch particles as a carrier for a variety of oilfield chemicals including tracers. Association of a tracer substance with a carrier is also disclosed in U.S. Pat. Nos. 7,032,662 and 7,347,260.
U.S. Pat. No. 6,645,769 proposes that multiple tracers should be located at respective zones of a reservoir during completion of a well and also proposes that individual tracers should be associated with carrier particles from which the tracers are eventually released into the reservoir and hence into fluid produced from the well. This document teaches that placing of tracers at an individual location during completion of the well may be achieved by immobilization on a filter or casing before that filter or section of casing is inserted into the well.